1. Field of the Invention
The invention relates generally to hardfacing coatings on a metallic work piece. In particular, the present invention relates to hardfacing coatings on drill bits.
2. Background Art
Rotary drill bits are generally well known in the art. These bits typically include three cone-shaped members adapted to connect to the lower end of a drill string. One example of such a drill bit is shown in FIG. 1. The bit 10 includes three individual arms 11 that extend downward from the bit body 19 at an angle with respect to the bit axis. The lower end of each arm 11 is shaped to form a spindle or bearing pin (shown as 16 in FIG. 2). A cone cutter 12, which includes a plurality of cutting elements 14, is mounted on each spindle and adapted to rotate thereon. As the drill string rotates, the cones 12 roll on the borehole bottom and rotate on about their respective spindles, thereby disintegrating the formation to advance the borehole.
FIG. 2 shows a partial, longitudinal cross section of a leg of a rock bit. Each leg includes a journal pin 16, on which a roller cone 12 is attached. During drilling, the roller cone 12 rotates around the journal pin 16. The rotation may cause the roller cone 12 to grind against the journal pin 16. Therefore, wear resistant materials are often included in critical areas on both the journal pin 16 and the inside of the roller cone 12 to minimize wear damage. In addition, bearing systems are provided to allow rotation of the cone cutter and serve to maintain the cone cutter on the spindle. These bearing systems may comprise roller bearings, ball bearings or friction bearings, or some combination of these.
As shown in FIG. 2, the journal pin 16 includes a cylindrical bearing surface having a hard metal insert 17 on a lower portion of the journal pin 16, while an open groove 18 is provided on the upper portion of the journal pin 16. Groove 18 may, for example, extend around 60% of the circumference of the journal pin 16, and the hard metal 17 can extend around the remaining 40%. The journal pin 16 also has a cylindrical nose 19 at its lower end.
The cavity (or inside surface) in the roller cone 12 typically contains a cylindrical bearing surface including an aluminum bronze insert 21 deposited in a groove in the steel of the roller cone 12 or as a floating insert in a groove in the roller cone 12. The aluminum bronze insert 21 in the roller cone 12 engages the hard metal insert 17 on the journal pin 16 and provides the main bearing surface for the roller cone 12 on the bit body. A nose button 22 is disposed between the end of the cavity in the roller cone 12 and the nose 19 of the journal pin and carries the principal thrust loads of the roller cone 12 on the journal pin 16. A bushing 23 surrounds the nose and provides additional bearing surface between the roller cone 12 and journal pin 16.
As shown in FIG. 2, a plurality of bearing balls 24 are fitted into complementary ball races in the cone and on the journal pin. The bearing surfaces between the journal pin and cone are lubricated by a grease composition. The balls 24 carry any thrust loads tending to remove the roller cone 12 from the journal pin 16 and thereby retain the roller cone 12 on the journal pin 16.
In addition, the interface between each spindle and its cone cutter may include a device (thrust bearing) to transmit thrust (axial) forces from the cone cutter to the spindle and thence to the bit. For description of various thrust bearings, see U.S. Pat. No. 5,868,502 issued to Cariveau et al. This patent is assigned to the assignee of the present invention and is incorporated by reference in its entirety.
The above described examples are greased bearing bits. The wear situation is even worse in non-lubricated open bearing bits. FIGS. 3 and 4 show partial, longitudinal cross sections of a leg of an open-bearing air bit. Referring to FIG. 3, a typical mining, roller bearing, air cooled rotary cone rock bit generally designated as 30, includes spindle 34 extending from the leg 33 forms bearing races 31 and 32 for roller bearings 35 and 36. Intermediate roller bearings 35 and 36, a plurality of ball bearings 37 rotatably retain the cone 38 on the spindle 34. Spindle 34 forms a radially disposed main bearing face 39 from which a spindle bearing 40 extends. A spindle thrust bearing disc, or “thrust button,” generally designated as 41, is pressed into a bearing cone cavity or socket 42 formed in cone spindle bearing 40. Cone 38 includes an internal cavity adapted to receive spindle 34 and the bearings 35, 36, and 37. The cone cavity includes cylindrical surfaces 43 and 44, ball bearing race 37a, and socket 45. The radial end face 46 of spindle bearing 40 extends into the cone cavity adjacent cylindrical surface 44. A cone thrust bearing disc, or “thrust button,” generally designated as 47, is pressed into a bearing cone cavity or socket 45 formed in cone 38. As discussed in greater detail below, cone thrust disc 47 engages spindle thrust disc 41, with the interface therebetween forming a thrust bearing.
Referring now to FIGS. 3 and 4, spindle 34 includes a main air fluid passage 48 formed in leg 33. Secondary air passages 49 direct air from main passage 48 to the main bearing face 50. An axially aligned air passage 51 directs air to a cross channel 52 that is formed in the radial end face 53 of the spindle 34. Cross channel 52 intersects and passes beneath, in this embodiment, a hardened steel bearing thrust button generally designated as 41 that is interference fitted or pressed into socket 45 formed in spindle 34. Air passes from central passage 51 into channel 52, thereby contacting base (not shown) of spindle thrust button 41. Air contacting base (not shown) of thrust button 41 serves to cool thrust button 41 and adjacent cone thrust button 47.
During operation of an open bearing, air bit, such as the one illustrated in FIGS. 3 and 4, the weight of the drill string places a load on the lower face of the cone 38. The axial component of this load generally causes contact between the radial end face or thrust face 46 of the spindle bearing 40 and the cone cavity or socket 45 formed in cone 38 on the lower, or load, side. The friction resulting from this contact between the cone 38 and the stationary support spindle 34 causes wear on the contacting surfaces that limits the useful life of the drill bit.
In greased bearing bits, the use of a lubricant on the contacting surfaces slows the rate of surface wear. However, in open bearing air bits, air is pumped through the drill pipe and through passages in the drill bit to the bearings for cooling and for keeping the bearings clean, rather than a lubricant. While air cools the outer roller bearings adequately, air cooling does not work as well in the nose area of the bit, which is subjected axial loads. The lack of lubrication and cooling on the thrust face increases heat generated by friction thereby promoting galling of the spindle and often causing premature failure of the spindle.
In addition to bearings and journal pins, the exposed, exterior parts of drill bits may also be subjected to wear. Some wear-susceptible exterior components of the drill bit include the exterior surfaces of the bit body, external surfaces of the cutting elements, and external surfaces of the roller cones on roller cone bits.
These parts, such as bit body, roller cones, and cutting elements, contact the formation during drilling and are subjected to abrasive actions. To prolong the life of a drill bit, these wear-prone surfaces should preferably be coated with a hardfacing material.
Various hardfacing materials methods are known in the art for minimizing wear on various parts of a drill bit. For example, U.S. Pat. Nos. 4,836,307 issued to Keshavan et al., and U.S. Pat. Nos. 5,944,127 and 6,659,206 both issued to Liang et al. disclose various hardfacing material compositions and particle size distributions suitable for use in hardfacing inserts, teeth, or roller cones. In addition, various methods have been developed for applying hardfacing coatings to wear prone surfaces on rock bits or inserts. These methods, for example, include thermal spraying, plasma arc welding, laser cladding, or other conventional welding methods.
Materials used in combination with the hardened steel surfaces in bit journal bearings, in provided, have included precipitation-hardened copper-beryllium (shown in U.S. Pat. Nos. 3,721,307 and 3,917,361), spinodally-hardened copper-tin-nickel (shown in U.S. Pat. No. 4,641,976), aluminum bronzes (shown in U.S. Pat. No. 3,995,917), and cobalt-based stellite alloys (shown in U.S. Pat. No. 4,323,284). These materials offer suitable ambient temperature yield strengths for use as structural elements or inlays, and acceptable anti-galling properties against hardened steel. However, at elevated PVs they can undergo a transition to high-friction operation, and except for the stellites, these alloys typically exhibit a rapid reduction in yield strength at temperatures above about 500° F. Because such high surface temperatures are not uncommon in bit thrust bearings, especially as drilling speeds have increased, if included on bit thurst surfaces, stellites have been the structural inlay material of choice for journal surfaces.
However, the effectiveness and durability of hardfacing depend on the compositions of the hardfacing materials. In addition, the compositions of the hardfacing materials also affect the strength of the bonding between the hardfacing layers and the underlying substrates. Most hardfacing compositions comprise wear-resistant particles (e.g., carbides) and a matrix metal (or alloy). Generally, altering a composition to enhance the wear resistance of the hardfacing overlay, typically results in a decrease of the fracture toughness of the overlay and reduction in the bonding strength between the hardfacing and the substrate. On the other hand, altering a composition to enhance the fracture toughness and bonding strength between the hardfacing and the substrate, typically results in a decrease in the wear resistance of the hardfacing overlay. Thus, the hardfacing materials used in the protection of drill bits or roller cones often represent a compromise between the desired properties, i.e., wear resistance, fracture toughness, and bonding strength.
Although the prior art hardfacing application techniques are capable of providing improved wear resistance to drill bits, there still exists a need for other techniques that can provide longer lasting drill bits.